Building Ruby with pthread is bad practice.

Ruby is not natively threaded; there is absolutely no reason to build against
pthread unless you intend to link against libraries themselves built with
pthread (tcl/tk). More information: http://blogs.sun.com/prashant/entry/ruby_and_enable_pthreads

Formula/ruby.rb

@@ -11,8 +11,7 @@ class Ruby <Formula
     system "./configure", "--prefix=#{prefix}",
                           "--disable-debug",
                           "--disable-dependency-tracking",
-                          "--enable-shared",
-                          "--enable-pthread"
+                          "--enable-shared"
     system "make"
     system "make install"
   end